Notes: For the same reason as above, I suggest to merge as much as possible into QEMU. It's not clear if things in this article are KVM-specific or can be applied to plain QEMU too. (Discuss in Talk:KVM#)

KVM, Kernel-based Virtual Machine, is a hypervisor built right into the Linux kernel. It is similar to Xen in purpose but much simpler to get running. To start using the hypervisor, just load the appropriate kvm kernel modules and the hypervisor is up. As with Xen's full virtualization, in order for KVM to work, you must have a processor that supports Intel's VT-x extensions or AMD's AMD-V extensions.

Using KVM, one can run multiple virtual machines running unmodified GNU/Linux, Windows, or any other operating system. (See Guest Support Status). Each virtual machine has private virtualized hardware: a network card, disk, graphics adapter, etc. See KVM Howto.

Differences among KVM, Xen, VMware, and QEMU can be found at the KVM FAQ.
KVM is used alongside Libvirt for better management with "virsh" command set. Check Libvirt for more details.

Get the packages

Arch Linux kernels provide the appropriate kernel modules to support KVM.
You can check if your kernel supports KVM with the following command (assuming your kernel is built with CONFIG_IKCONFIG_PROC):

$ zgrep KVM /proc/config.gz

KVM requires that the virtual machine host's processor has virtualization support (named VT-x for Intel processors and AMD-V for AMD processors). You can check whether your processor supports hardware virtualization with the following command:

$ lscpu

You processor supports virtualization only if there is a line telling you so.

You can also run:

$ grep -E "(vmx|svm)" --color=always /proc/cpuinfo

If nothing is displayed after running that command, then your processor does not support hardware virtualization, and you will not be able to use KVM.

Set up kernel modules

First, you need to add your user account into the kvm group to use the /dev/kvm device.

# gpasswd -a <Your_Login_Name> kvm

Note: If you use systemd and are a local user, this is not necessary, as access is now granted by systemd/udev.

Secondly, you have to choose one of the following depending on the manufacturer of the VM host's CPU.

If you have Intel's VT-x extensions, modprobe the kvm_intel module.

# modprobe kvm_intel

If you have AMD's AMD-V (code name "Pacifica") extensions, modprobe the kvm-amd module.

# modprobe kvm_amd

If modprobing kvm_intel or kvm_amd fails but modprobing kvm succeeds, (and lscpu claims that hardware acceleration is supported), check your BIOS settings. Some vendors (especially laptop vendors) disable these processor extensions by default. To determine whether there's no hardware support or there is but the extensions are disabled in BIOS, the output from dmesg after having failed to modprobe will tell.

How to use KVM

As qemu-kvm package is merged into qemu, see the main article QEMU, and especially the section Enabling KVM.

Paravirtualized guests (virtio)

KVM offers guests the ability to use paravirtualized block and network devices, which leads to better performance and lower overhead.
For Windows, a paravirtualized network driver can be obtained here.
FreeBSD has the ability to use virtio drivers since 10.0 (unreleased). A backport of the drivers are available in the port emulators/virtio-kmod for FreeBSD 8.3 and 9.0.

Note:-boot order=c is absolutely necessary when you want to boot from it. There is no auto-detection as with -hd*.

Almost the same goes for the network:

$ qemu-kvm -net nic,model=virtio

Preparing an (Arch) Linux guest

To use virtio devices after an Arch Linux guest has been installed, the following modules can be loaded in the guest: virtio, virtio_pci, virtio_blk, virtio_net, and virtio_ring (for 32-bit guests, the specific "virtio" module is not necessary).

If you want to boot from a virtio disk, the initial ramdisk must be rebuilt. Add the appropriate modules in /etc/mkinitcpio.conf like this:

MODULES="virtio_blk virtio_pci virtio_net"

and rebuild the initial ramdisk:

# mkinitcpio -p linux

Virtio disks are recognized with the prefix v (e.g. vda, vdb, etc.); therefore, changes must be made in at least /etc/fstab and /boot/grub/grub.cfg when booting from a virtio disk.

Tip: When referencing to disks by UUID in both /etc/fstab and bootloader, nothing has to be done.

Windows will detect the fake disk and try to find a driver for it. If it fails, go to Device Manager, locate the SCSI drive with an exclamation mark icon (should be open), click "Update driver" and select the virtual CD-ROM. Don't forget to mark the checkbox which says to search for directories recursively.

When the installation is successful, you can turn off the virtual machine and launch it again, now with the virtio driver.

$ qemu-kvm -drive file=windows.img,if=virtio -m 512 -vga std

Note: If you encounter the Blue Screen of Death, make sure you did not forget the -m parameter, and that you do not boot with virtio instead of ide for the system drive before drivers are installed.

Note: The flag "boot=on" was removed due to newer versions no need it anymore.

Then install the virtio drivers from the disk you downloaded; Go to the Device Manager, locate the network adapter with an exclamation mark icon (should be open), click "Update driver" and select the virtual CD-ROM. Don't forget to mark the checkbox which says to search for directories recursively.

Preparing a FreeBSD guest

Install the emulators/virtio-kmod port if you are using FreeBSD 8.3 or later up until 10.0-CURRENT where they are included into the kernel. After installation, add the following to your /boot/loader.conf file:

And verify that /etc/fstab is consistent. If anything goes wrong, just boot into a rescue CD and copy /etc/fstab.bak back to /etc/fstab.

Enabling KSM

Kernel Samepage Merging (KSM) is a feature of the Linux kernel introduced in the 2.6.32 kernel. KSM allows for an application to register with the kernel to have its pages merged with other processes that also register to have their pages merged. For KVM, the KSM mechanism allows for guest virtual machines to share pages with each other. In an environment where many of the guest operating systems are similar, this can result in significant memory savings.

There should be a /sys/kernel/mm/ksm/ directory containing several files. You can turn KSM on or off by echoing a 1 or 0 (respectively) to /sys/kernel/mm/ksm/run:

# echo 1 > /sys/kernel/mm/ksm/run

Or set it up by creating the file /etc/tmpfiles.d/ksm.conf:

w /sys/kernel/mm/ksm/run - - - - 1

If KSM is running, and there are pages to be merged (i.e. more than one similar VM is running), then /sys/kernel/mm/ksm/pages_shared should be non-zero. From the kernel documentation in Documentation/vm/ksm.txt:

The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
pages_shared - how many shared unswappable kernel pages KSM is using
pages_sharing - how many more sites are sharing them i.e. how much saved
pages_unshared - how many pages unique but repeatedly checked for merging
pages_volatile - how many pages changing too fast to be placed in a tree
full_scans - how many times all mergeable areas have been scanned
A high ratio of pages_sharing to pages_shared indicates good sharing, but
a high ratio of pages_unshared to pages_sharing indicates wasted effort.
pages_volatile embraces several different kinds of activity, but a high
proportion there would also indicate poor use of madvise MADV_MERGEABLE.

An easy way to see how well KSM is performing is to simply print the contents of all the files in that directory.

Enable HugePages

You may also want to enable hugepages to improve the performance of your virtual machine.
With an up to date Arch Linux and a running KVM you probably already have everything you need. Check if you have the directory /dev/hugepages. If not create it.
Now we need the right permissions to use this directory. Check if the group kvm exist and if you are member of this group. This should be the case if you already have a running virtual machine.

$ getent group kvm

kvm:x:78:USERNAMES

Add to your /etc/fstab:

hugetlbfs /dev/hugepages hugetlbfs mode=1770,gid=78 0 0

Of course the gid must match that of the kvm group. The mode of 1770 allows anyone in the group to create files but not unlink or rename each other's files. Make sure /dev/hugepages is mounted properly:

Now you can calculate how many hugepages you need. Check how large your hugepages are:

$ cat /proc/meminfo | grep Hugepagesize

Normally that should be 2048 kB ≙ 2 MB. Let's say you want to run your virtual machine with 1024 MB. 1024 / 2 = 512. Add a few extra so we can round this up to 550. Now tell your machine how many hugepages you want:

# echo 550 > /proc/sys/vm/nr_hugepages

If you had enough free memory you should see:

$ cat /proc/meminfo | grep HugePages_Total

HugesPages_Total: 550

If the number is smaller, close some applications or start your virtual machine with less memory (number_of_pages x 2):

Bridged Networking

Using netcfg

Bridged networking is used when you want your VM to be on the same network as your host machine. This will allow it to get a static or DHCP IP address on your network, and then you can access it using that IP address from anywhere on your LAN. The preferred method for setting up bridged networking for KVM is to use the netcfg package. You will also need to install bridge-utils.

Live snapshots

A feature called external snapshotting allows one to take a live snapshot of a virtual machine without turning it off. Currently it only works with qcow2 and raw file based images.

Once a snapshot is created, KVM attaches that new snapshotted image to virtual machine that is used as its new block device, storing any new data directly to it while the original disk image is taken offline which you can easily copy or backup. After that you can merge the snapshotted image to the original image, again without shutting down your virtual machine.

At this point, you can go ahead and copy the original image with cp -sparse=true or rsync -S.
Then you can merge the original image back into the snapshot.

# virsh blockpull --domain archey --path /vms/archey.snapshot1

Now that you have pulled the blocks out of original image, the file /vms/archey.snapshot1 becomes the new disk image. Check its disk size to see what it means. After that is done, the original image /vms/archey.img and the snapshot metadata can be deleted safely. The virsh blockcommit would work opposite to blockpull but it seems to be currently under development in qemu-kvm 1.3 (including snapshot-revert feature), scheduled to be released sometime next year.

This new feature of KVM will certainly come handy to the people who like to take frequent live backups without risking corruption of the file system.

Poor Man's Networking

Setting up bridged networking can be a bit of a hassle sometimes. If the sole purpose of the VM is experimentation, one strategy to connect the host and the guests is to use SSH tunneling.

The basic steps are as follows:

Setup an SSH server in the host OS

(optional) Create a designated user used for the tunneling (e.g. tunneluser)